Recent advances in plant genetic engineering have opened new doors to engineer plants to have improved characteristics or traits, such as plant disease resistance, insect resistance, herbicidal resistance, yield improvement, improvement of the nutritional quality of the edible portions of the plant, and enhanced stability or shelf-life of the ultimate consumer product obtained from the plants. Thus, a desired gene (or genes) with the molecular function to impart different or improved characteristics or qualities, can be incorporated properly into the plant's genome. The newly integrated gene (or genes) coding sequence can then be expressed in the plant cell to exhibit the desired new trait or characteristics. It is important that appropriate regulatory signals must be present in proper configurations in order to obtain the expression of the newly inserted gene coding sequence in the plant cell. These regulatory signals typically include a promoter region, a 5′ non-translated leader sequence and a 3′ transcription termination/polyadenylation sequence.
A promoter is a non-coding genomic DNA sequence, usually upstream (5′) to the relevant coding sequence, to which RNA polymerase binds before initiating transcription. This binding aligns the RNA polymerase so that transcription will initiate at a specific transcription initiation site. The nucleotide sequence of the promoter determines the nature of the enzyme and other related protein factors that attach to it and the rate of RNA synthesis. The RNA is processed to produce messenger RNA (mRNA) which serves as a template for translation of the RNA sequence into the amino acid sequence of the encoded polypeptide. The 5′ non-translated leader sequence is a region of the mRNA upstream of the coding region that may play a role in initiation and translation of the mRNA. The 3′ transcription termination/polyadenylation signal is a non-translated region downstream of the coding region that functions in the plant cell to cause termination of the RNA synthesis and the addition of polyadenylate nucleotides to the 3′ end.
It has been shown that certain promoters are able to direct RNA synthesis at a higher rate than others. These are called “strong promoters”. Certain other promoters have been shown to direct RNA synthesis at higher levels only in particular types of cells or tissues and are often referred to as “tissue specific promoters”, or “tissue-preferred promoters” if the promoters direct RNA synthesis preferably in certain tissues but also in other tissues at reduced levels. Certain promoters are able to direct RNA synthesis at relatively similar levels across all tissues of a plant. These are called “constitutive promoters” or “tissue-independent” promoters. Constitutive promoters can be divided into strong, moderate and weak according to their effectiveness to direct RNA synthesis. In some cases promoters are able to direct RNA synthesis when they are induced by external stimuli such as chemicals, stress, or biotic stimuli. These are called “inducible promoters”.
The ability to externally control the expression of selected genes and thereby their gene products in plant cells and/or field grown plants can provide important agronomic and foodstuff benefits. This control is desirable for the regulation of genes that might be placed into transgenic plants and has many applications including, but not limited to, (1) prolonging or extending the accumulation of desirable nutritional food reserve in seeds, roots, (2) producing and accumulating products in plant tissues at a defined time in the developmental cycle such that these products are convenient for harvest and/or isolation, and (3) initiating the expression a pest-specific toxin at the site of pathogen attack. There is an ongoing interest in the isolation of novel inducible promoters which are capable of controlling the expression of a chimeric gene or (genes) at certain levels in a plant cell when exposed to external stimuli.